The present invention pertains generally to filters and methods for separating ions of relatively low mass to charge ratios from ions of relatively high mass to charge ratios. More particularly, the present invention pertains to filters and methods for separating metal ions from a feed source material that includes metal and non-metallic atoms. The present invention is particularly, but not exclusively, useful for separating light metal ions from heavy metal ions in the presence of other gaseous components in the feed.
It is known that the constituent elements of a multi-species plasma can be separated from each other in several different ways. One effective way to do this is to separate ions of the elements from each other according to their respective mass to charge ratios. A device and method for this purpose has recently been disclosed in U.S. Pat. No. 6,096,220, which issued to Ohkawa for an invention entitled xe2x80x9cPlasma Mass Filter.xe2x80x9d
This patent is assigned to the same assignee as the present invention, and is incorporated herein by reference for examples of a device and a method for processing a multi-species plasma to separate the ions of a heavy metal from the ions of a light metal.
As a practical matter, the source material for a multi-species plasma that contains both heavy metals and light metals, will contain more than just the metals. Typically, the source material (or feed) will include compounds such as oxides, hydroxides, chlorides or fluorides of the metals. Consequently, in order to fully ionize a source material, it is necessary to ionize all of the constituent elements; both metals and nonmetals. Different elements in a plasma, however, ionize at different electron temperatures. Stated differently, different elements have different ionization potentials.
By definition, the ionization potential of an element is the energy, expressed as electron volts (eV), that is required to detach an electron from a neutral atom. For gaseous elements (e.g. oxygen, chlorine and fluorine) the ionization potentials are relatively high and are between twelve and eighteen electron volts (12-18 eV). On the other hand, the ionization potentials for metals are relatively low and are in a range from four to eight electron volts (4-8 eV).
As a practical matter, the ionization of atoms in a plasma will begin to occur when electrons in the plasma have been heated to an electron temperature that is below the ionization potential of the atoms. This happens because heated electrons will evolve to a Maxwellian distribution at the electron temperature. Thus, for a given temperature, many of the electrons will have higher energy than indicated by the electron temperature. It is these energetic electrons that then do most of the ionization.
The efficacy of a plasma mass filter, such as the one disclosed by Ohkawa and referenced above, relies solely on the ionization of metals in the source material (feed). It does not matter whether gaseous elements in the source material have been ionized. A consequence of this is that, since only the metal elements need to be ionized, lower electron temperatures can be used. Furthermore, energy savings are significant since metal atoms will normally amount to less than half of the atoms in a typical source material and a plasma with lower electron temperature radiates less energy. Importantly, a partially ionized plasma (i.e. one wherein the metals have been ionized, but the gaseous elements of the source material have not) can still be effectively processed in a plasma mass filter for the purpose of separating metal ions from each other according to their respective mass to charge ratios. One caveat here is that the density of the gaseous elements (i.e. neutrals) in the chamber of an operational plasma mass filter may need to be controlled so as not to erode the separation quality.
In light of the above it is an object of the present invention to provide a device and method for separating ions in a partially ionized plasma according to mass to charge ratios that maintain separation quality during their operation. It is another object of the present invention to provide a device and method for separating ions in a partially ionized plasma according to mass to charge ratios that increases the effective throughput. Yet another object of the present invention is to provide a device and method for separating ions in a partially ionized plasma according to mass to charge ratios that reduces processing costs by reducing both the energy cost per ion and the fraction of atoms that need to be ionized. Still another object of the present invention is to provide a device and method for separating ions in a partially ionized plasma according to mass to charge ratios that is easy to use, relatively simple to manufacture and comparatively cost effective.
In accordance with the present invention, a partially ionized plasma mass filter includes a substantially cylindrical shaped chamber that defines a longitudinal axis. Magnetic coils are mounted on the chamber to establish a magnetic field (B) in the chamber that is oriented substantially parallel to the chamber axis. Additionally, electrodes are mounted on the chamber to establish an electric field (E) in the chamber that is oriented substantially perpendicular to the chamber axis. Preferably, the electric potential is a positive along the axis and substantially zero at the wall of the chamber. Thus, crossed electric and magnetic field (Exc3x97B) are established inside the chamber.
An injector is provided for introducing a feed source material into the chamber along the axis. Specifically, the feed includes metal atoms and non-metallic atoms. For example, the feed may contain metallic oxides, hydroxides, chlorides or fluorides. Importantly, the present invention exploits the fact that the metals have lower ionization potentials than do oxygen or other gaseous elements (e.g. halogens such as fluorine or chlorine). Stated differently, the metals in the feed will have ionization potentials that are in a low range (e.g. 4-8 eV), and the gas atoms will have an ionization potential that is in a high range (e.g. 12-18 eV).
With the above in mind, an antenna is mounted on the chamber to generate an electron temperature that is slightly below the low range, but substantially below the high range. The consequence of this is that the feed source material is only partially ionized. Specifically, the metal atoms are dissociated from the gas atoms and ionized, but the non-metallic atoms are not ionized. Instead, the non-metallic atoms remain as a neutral gas. Specifically, as envisioned by the present invention the metal atoms will include both light and heavy metals. Thus, the metal atoms will ionize into light ions having a relatively low mass to charge ratio (M1) and heavy ions having a relatively high mass to charge ratio (M2).
The crossed electric and magnetic fields inside the chamber will influence the light ions (M1) and the heavy ions (M2) to travel on trajectories that differ according to the mass to charge ratio of the respective ions. Thus, the light ions are separated from the heavy ions. Accordingly, a first collector can be positioned in the chamber to collect the light ions (M1), and a second collector can be positioned in the chamber to collect the heavy ions (M2). In a preferred embodiment of the present invention, the first collector is positioned at an end of the chamber, opposite the injector, to collect the light ions after they have passed through the chamber. In this embodiment, the wall of the chamber between its two ends will serve as the second collector.
In addition to the structure disclosed above for the filter of the present invention, the filter may include a vacuum pump that is connected in fluid communication with the chamber. The function of this vacuum pump is to remove gas atoms (e.g. oxygen, fluorine, chlorine) as they collect near the wall of the chamber. Further, these gas atoms can then be directed back into the chamber to recombine with the light ions and the heavy ions to reform metal oxides (or hydroxides, fluorides, chlorides) which may be more easily removed from the chamber.